Fluorescent screens



Oct. 6, 1959; s. H. PATTEN FLUORESCENT SCREENS Filed May '3, 1957 I Poly(methyl methacrylotg) IOO /o -FlG-l /WM%\ WWW 1 I00% |0 (QC/lo) Vinyl chloride/ vinyl acetate Po yflsobuty mefl'mcl ycopolymer lcf e) Vinyl chloride/vinyl acetate copolymer, 3 Poly (methyl methocrylafe) and Po|y(isobufyl methccryloie) I \'Calcium fungstufecellulose acetate Support "BY 75mm ATTORNEY United States Patent FLUORESCENT SCREENS Stanley Hancock Patten, Towanda, Pa., assignor to E. I. du Pont de Nemours and Company, Wilmington, Del., a corporation of Delaware Application May 3, 1957, Serial No. 656,807

6 Claims. (Cl. 250-80) This invention relates to luminescent screens having a protective layer. More particularly, it relates to X-ray intensifying screens having a protective layer or coating for the layer of fluorescent material. Still more particularly, it relates to such screens wherein the protective layer is composed of a mixture of (1) a vinyl chloride/ vinyl acetate copolymer with (2) a poly(alkyl acrylate) or poly(alkyl methacrylate) wherein the alkyl group contains 1 to 2 carbon atoms and (3) such a polymer where'the alkyl group contains 4 to 6 carbon atoms.

Nearly all commercial X-ray intensifying screens have protective coatings for the fluorescent layer. These coatings vary widely in composition and structure. They offer varying degrees of protection toward abrasion and scratching or other forms of physical surface marking. Usually these protective coatings or layers are composed of cellulose derivatives and homopolymers but, insofar as applicant is aware, the prior art coatings suffer the disadvantage that they are somewhat permeable to moisture and become stained by contact with photographic processing solutions, e.g., developer solutions. These solutions penetrate into the phosphor layer where they become impervious to removal by surface cleaning. This has the disadvantage that after a short period of time the developer stain attains high color which absorbs a major portion of the light emanating from the screen on excitation with X-radiation. This results in a radiograph bearing an area of low photographic density corresponding in size and shape to the stained portion of the X-ray intensifying screen. Such spots interfere with accurate radiographic diagnosis and the damaged screen must be discarded.

A further disadvantage of the commercially available X-ray intensifying screens is that the protective-coatings are susceptible to static charge build-up during normal use. The degree of static susceptibility is primarily determined by the chemical composition of the protective coating, different materials exhibiting different susceptibilities. Any surface may become charged with static electricity when it is brought into contact with another surface bearing a static charge or when the subject surface is abraded with a material of similar or dissimilar composition. Thus, X-ray intensifying screens become charged on loading or unloading the screen cassette with photographic film. Discharge of this static electricity, by any means whatever, causes exposure of the lightsensitive film and, as a consequence, damaging marks characteristic of the static electricity appear on the developed radiograph.

An object of this invention is to provide improved luminescent screens. Another object is to provide such screens which have protective coatings possessing improved resistance to abrasive forces, scratching and improved antistain characteristics. Afurther object is to provide X-ray intensifying screens and fluoroscopic screens which have increased resistance to static charge build-up. A still further object is to provide such screens;

with improved protective layers which do not affect the physical and radiological properties of the fluorescent or' phosphorescent layer. Still other objects will be appar-. ent from the following description of the invention.

It has been found in accordance with this invention that the above objects can be attained by providing the active luminescent layer of a luminescent screen with a thin protective film or layer composed of an intimate mixture of components (1) a vinyl chloride/ vinyl acetate copolymer containing at least 60% by Weight of the former, (2) a poly(alkyl acrylate) or poly(alkyl methacrylate) Wherein said alkyl radical contains 1 to 2 carbon atoms, and (3) a poly(alkyl acrylate) or poly(alkyl methacrylate) wherein said alkyl group contains 4 to 6 carbon atoms, predominating in the first component and containing at least 5% by weight of each of the latterf components. Thus, component (1) can range from 40% to 90%, component (2) can range from 45% to 5% and component (3) can range from 45 to 5%, all by Weight, of the total weight of the copolymer. The novel coatings have a composition which lies within the area depicted in the ternary diagram of Fig. 1 of the attached drawing which consitutes a part of this application. The three points defining this triangular area are according to the figure: (I) 50% by Weight vinyl chlo ride/vinyl acetate copolymer, 45 poly(methyl methacrylate), 5% poly(isobutyl methacrylate), (II) 90% vinyl chloride/vinyl acetate copolymer, 5% poly(methyl methacrylate), 5% poly(isobutyl methacrylate), and (III) 50% vinyl chloride/vinyl acetate copolymer, 5% poly(methyl methacrylate), 45% poly(isobutyl methacrylate). While any composition as defined above may be used, the preferred mixture that excels all others contains 50% vinyl chloride/vinyl acetate copolymer, 25 poly(methyl methacrylate), and 25% poly(isobutyl methacrylate) all percentages by weight.

The protective coating is applied to the surface of the luminescent screen, e.g., an X-ray intensifying screen, from solution in an organic solvent for the resins or copolymers. Suitable solvents include alcohols, e.g., cyclohexanol, and aliphatic ketones, e.g., acetone, methyl ethyl ketone, diethyl ketone and methyl isobutyl ketone, cyclohexanone; esters, e.g., butyl acetate and amyl acetate, and mixtures of two or more of these solvents, or mixtures of the foregoing solvents with such diluents as aromatic hydrocarbons, for example, benzene, toluene and the xylenes. Best results are obtained when the mixture of polymers is dissolved in acetone or 1/1 methyl ethyl ketone/ toluene mixture. The concentration of the solution employed is limited only by the solubility lim its of the mixture of polymers, this being in the neighbor-I hood of 18 to 20% by Weight. The solution should have sufiicient solids content to yield a workable viscosity and for this reason it has been found that the solids content should be at least 7% by weight.

An exemplary procedure for making an X-ray intensify! ing screen bearing the improved protective coating is as follows: A sheet of material which is to serve as a support for the X-ray intensifying screen is placed on a smooth casting surface, e.g., plate glass. The support may be cardboard or a polymeric material or films in sheet form, e.g., cellulose derivatives, polyesters or other superpolymers. The sheet support is adhered to the glass by means of a pressure-sensitive flexible tape or .a coating of pressure-sensitive adhesive. A suspension of the active phos phor in a suitable binder is cast at the proper thickness on the cardboard or similar support and allowed to dry,

Next, an overcoating of the mixture of polymers described above is applied to the phosphor layer from solution in an organic solvent. The protective coating is usually 0.005 to 0.015 inch in thickness, as applied, and dries to form a continuous film approximately of said thickness. Drying of the protective coating is carried out at room temperature during an initial stage which is followed by oven-drying at an elevated temperature, e.g., about 60 C. to" 80 C. Finally, the coated composite screen is stripped from the glass plate and trimmed to the desired size. 7

In place of a cardboard support which may be suitably sized or coated, e.g., with baryta, there may be used cellulose acetate, cellulose propionate, cellulose acetate propionate and cellulose acetate butyrate; polyvinyl chloride, poly(vinyl chloride co vinyl acetate); polyamides or nylon, e.g., polyhexamethyleneadipamide; polyesters, e.g., polyethylene terephthalate, and metal sheets, e.g., aluminum and aluminum alloys. The organic supports may, if desired, contain an opacifying agent or pigment, e.g., titanium dioxide, magnesium oxide or chalk, or be coated with such a material in a suitable binder. In the case of X-ray intensifying screens, the support should be permeable to X-rays.

In testing the screens for anti-static properties, the following test procedure is satisfactory. A pair of candidate screens 5 inches by 7 inches in size is thoroughly dried in an oven maintained at 60 C. Upon removal from the oven and immediately after cooling to room temperature, the screens are taken into the photographic dark room, where the temperature is 72 to 75 F. and the relative humidity about 40%. Here the screens are placed face-to-face and a piece of double-coated X-ray sensitive photographic film having a cellulose acetate film base is inserted between them. While moderate pressure is exerted on the screen-film assembly, the X-ray film is rapidly withdrawn from between the screens. This process is repeated ten times and then the film is developed in the normal manner. The X-ray film from the candidate screens is compared for prominence of static markings to a similar film treated in a similar manner by use of a pair of control screens bearing cellulose acetate protective coatings.

The following procedure has been found to be satisfactory for the determination of developer-stain resistance. A pair of screen samples bearing the above protective coating and measuring approximately 2 inches by 4 inches in size is thoroughly cleaned with alcohol and then mounted in a conventional cassette. An X-ray sensitive photographic film is placed between the screen samples,

the cassette 1s closed to render it light-proof, and it is exposed to X-radiation to produce a photographic density of approximately unity on the film. The radiograph so produced serves as a control for comparison with subsequent fadiographs made with stained screens. Each screen is now stained by placing a drop of freshly prepared photographic developing solution at the center of their areas and on the surfaces comprising the protective coating. The spotted screens are allowed to dry and age for a period of 48 hours at room temperature and in isolation from actinic light. At the end of this time, the dried, encrusted developer solution is rinsed from the screen surface with a stream of warm water. The screen surface is washed further with soap-and-water, rinsed with water and finally washed with ethanol. Care should be exercised to avoid scratching or marring the surface during the cleaning operations. The screens are allowed to age for a period of three weeks, after which time another radiograph is taken of the stained screens. This iadiograph is examined and compared with its predecessor for areas of low photographic density denoting the 'presence of developer staining.

The invention will be further illustrated by but is not limited to the following examples, wherein the parts stated are by weight.

4 Example I A protective coating solution is prepared by admixing the following components:

(/ 10) vinyl chloride/ vinyl acetate copolymer Methyl ethyl ketone 504 The solid polymers are added to the prescribed amount of solvent contained in a suitable vessel and the whole milled for 16-20 hours to effect solution. The solution is filtered through bolting cloth in order to remove any undissolved polymer. A suitably sized, smooth-surfaced cardboard support 1 as shown in Fig. 2 of the drawing, said support being from 0.010 to 0.020 inch in thickness, is placed on the surface of a glass plate and brought into uniform surface contact therewith by the aid of a pressuresensitive adhesive. A layer 2 of fluorescent calcium tungstate in a cellulose acetate or cellulose nitrate binder is coated onto the smooth surface of said support and dried. The dry layer of fluorescent calcium tungstate is then overcoated with the methyl ethyl ketone solution of the mixture of polymers to form a protective layer 3 having a wet-thickness of 0.012 inch. The layer is allowed to dry at room temperature for 16 hours and is then dried at 60 C. for 2 hours. Finally, the composite screen is stripped from the glass plate and trimmed to the desired size. Control screens were made in like manner except that a butyl acetate/ acetone solution of cellulose acetate was used to make the protective coating.

On testing according to the procedures outlined earlier in this application, the screens were found to have complete resistance against staining with fresh developer solution and to be free of static markings. On the other hand, the control screens bearing cellulose acetate protective surfaces showed distinct injurious staining by the developer and accumulated moderate amounts of static electricity.

Example II A resin solution is made according to the following formulation:

An X-ray intensifying screen is fabricated according to the procedure described in the preceding example and consists of the fluorescent layer and cardboard support. A protective coating is applied to the surface of the phosphor layer from the foregoing acetonemethyl ethyl ketone solution with a doctor-knife at a wet thickness of 0.005 inch. The screen is dried for 30 minutes at 78 F. and then baked for 2 hours at F. The finished screen is cut to the desired size.

The screen so formed was found to be completely resistant to staining with fresh developer solution and to be substantially non-susceptible to static charge accumulation. It had good anti-abrasion characteristics and had good scratch-resistance.

Example 111 A solution is made according to the following formulatio'n:

This solution of polymers is applied to an X-ray intensifying screen according to the procedure described in Example IV A solution is made according to the following formulation:

' Grams (90/10) vinyl chloride/vinyl acetate copolymer 54 Poly (methyl methacrylate) 9 Poly(isobutyl methacrylate) 27 Methyl ethyl ketone 510 After applying this coating solution to the surface of an X-ray intensifying screen as described in Example I, a significant improvement was found in the developer stain resistance and static susceptibility of the screen.

Example V A solution is made according to the following formulation:

After applying this coating solution to the surface of an X-ray intensifying screen according to the method described in Example I, a marked improvement was found with respect to the developer resistance and static susceptibility of the screen.

All of the preceding examples have specified the use of the new resinous compositions as protective coatings applied to the front surface of the screen structure. Other uses for these coating compositions are: (1) for coatings to be applied to the back surfaces of X-ray intensifying screens to improve moisture impermeability and prevent warping, and (2) as an edging for X-ray intensifying screens. This latter application prevents delamination of the screen which might otherwise occur during handling.

Any of the aforementioned compositions may be used for these additional purposes. Following is an example related to X-ray intensifying screen back-coating:

Example VI An X-ray intensifying screen consisting of a cardboard or cellulose acetate film support, an active phosphorcontaining layer and a protective coating is fabricated according to methods known to the art. The protective coating may be composed of one of the above compositions.

A solution to be applied to the back surface of the screen is formulated as follows:

Grams (90/ vinyl chloride/vinyl acetate copolymer 15.00 Poly(methyl methacrylate) 7.50 Poly(isobutyl methacrylate) 7.50 Acetone 68.00 Methyl ethyl ketone 68.00 Toluene 34.00

The X-ray screens having the foregoing supports are made to lie flat and face-down upon a plane casting surface, e.g., a sheet of glass. Using a conventional doctor-knife technique, the back surface of the support is overcoated with a by weight solution of cellulose nitrate dissolved in n-butyl acetate. After drying for 2 hours at room temperature, the back surface of the screen is recoated with the above solution at a wet thickness of 0.008 inch. This is allowed to dry for 2 hours at room temperature and an additional 2 hours at 140 F. The X-ray intensifying screen so prepared is found to be impervious to moisture penetration and resistant to warping when exposed to humid atmospheres for long periods of time.

6 l Example VII The solution is mixed and filtered as in Example I. On a support such as is described in Example I is coated a layer of a fluorescent barium lead sulfate in a cellulose nitrate binder and the layer is dried. The dry layer is then overcoated with the acetone/methyl ethyl ketone solution of the mixture of polymers to form a protective layer having a wet thickness of 0.005 inch. The layer is allowed to dry in air for fifteen minutes and at 60 C. for 2 hours. The finished screen is cut to the desired size. The screen so formed was found to be highly resistant to staining with fresh developer solution accumulation.

Example VIII A sheet of 'biaxially oriented polyethylene terephthalate film coated on one surface with a substratum of poly vinylidene chloride co methacryl ate co itaconic acid) Was prepared as described in Alles and Saner, U.S., 2,698; 240. The substratum was coated in turn with a reflective layer 0.0005 inch thick comprising titanium dioxide in a chlorosulfonated polyethylene binder and then with a phosphor layer 0.0045 inch thick comprising fluorescent calcium tungstate in a chlorosulfonated polyethylene binder after the manner described in assignees Alles application Ser. No. 512,282 filed May 31, 1955, now Patent No. 2,819,183. These coatings were made from toluene dispersions, and the coatings Were dried by passing the sheet through a hot-air oven. The dried phosphor layer Was then coated with the following solution of polymers:

. Grams (/10) vinyl chloride/vinyl acetate copolymer 45 Poly(methyl methacrylate) (as a 40% solution in 33.75 g. methyl ethyl ketone) 22.5 Poly(isobutyl methacrylate) 22.5v Acetone 250 Methyl ethyl ketone 250 Toluene by a knife coater 0.005 inch from the surface. The coating was dried for 17 minutes in a hot-air dryer at temperatures increasing from 120l40 F., and then baked for two hours at F. No samples of the coated phosphor screen thus prepared showed any staining by developer in the developer stain test described above.

Example IX The procedure of Example VIII was repeated except that the final coating solution was made as follows:

. Grams (90/10) vinyl chloride/vinyl acetate copolymer 229 Poly(methyl methacrylate) (as a 40% solution in 172.6 g. methyl ethyl ketone) 114 Poly(isobutyl methacrylate) 114 Acetone 723 Methyl ethyl ketone 1686 screens, i.e., abrasion resistance, flexibility, lack of color, stability to ultraviolet light, resistance to degradation by X-radiation, cleanability, and lack of brittleness, and they exhibited normal speed and resolving power. In place of the specific protective coating solutions set forth in the foregoing examples, there may be substituted various other solutions of mixtures of polymers having the composition set forth above. Thus, in place of the poly(rnethyl methacrylate) component there may be substituted poly(methyl acrylate), poly(ethyl acrylate) or poly(ethyl methacrylate) in an amount from to 45% by Weight of the total amount of polymers in the composition.

Similarly, in place of the poly(isobutyl rnethacrylate) there may be substituted poly(isobutyl acrylate), poly- (butyl acrylate), poly(butyl methacrylate), poly(n-h'exyl acrylate) or poly(n-hexyl methacrylate) in an amount from 5% to 45% by weight of the total amount of polymers in the composition.

Similarly, screens having other supports and other fluorescent materials as well as other binding agents including cellulose nitrate, the poly(alkyl acrylates) and poly(alkyl methacrylates) described above, or chlorosulfonated polyethylene binding agents, can be provided similarly with the above-described protective layers.

In place of the particular fluorescent materials described [III the foregoing examples there may be substituted equivalent amounts of other fluorescent compounds or mixtures of compounds which are suitable for fluorescent screens. Additional suitable materials include zinc sulfide, zinc silicate, mixed crystals. of Zinc sulfide and cadmium sulfide, zinc oxide and calcium silicate, zinc phosphate, alkali halides, cadmium sulfide, cadmium selenide, zinc selenide, zinc telluride, cadmium telluride, cadmium tungstate, magnesium fluoride, Zinc fluoride and strontium sulfide.

The protective coatings described herein are used in relation with luminescent screens of all types but particularly with X-ray intensifying and fluorescent screens to protect the delicate phosphorogen layer from physical abuse and damage.

The new and improved screens having protective coatings that have been described above are of advantage over previous coated screens in that they are relatively impermeable to moisture and therefore protect the screen, and more particularly the fluorescent layer, from staining. If the screen comes into contact with photographic developing solution, for example, the solution may be washed away and damaging staining does not result. Another advantage of the new screens is the resistance of the protective coatings towards static charge build-up. Both of these advantages are helpful in prolonging the useful life of an X-ray intensifying screen.

I claim:

1. A luminescent screen comprising a support and a layer of luminescent material bearing a protective layer comprising a mixture of components (1) vinyl ch10- ride/vinyl acetate copolymer containing at least by weight of the former, (2) a polyester taken from the group consisting of poly(alkyl acrylate) and poly(alkyl methacrylate) wherein the alkyl radical contains 1 to 2 carbon atoms and (3) a polyester taken from the group consisting of poly(alkyl acrylate) and poly(alkyl methacrylate) wherein said alkyl radical contains 4 to 6 carbon atoms, the first component being present in an amount from 40% to and the latter each being present in an amount from 5% to 45%, by weight, of said mixture.

2. A screen as set forth in claim 1 wherein the first component constitutes about 50% and the latter component constitutes about 25%, each by weight, of the mixture.

3. A screen as set forth in claim 1 wherein said support is cardboard.

4. A screen as set forth in claim 1 wherein said support is a hydrophobic polymeric film base.

5. A screen as set 'forth in claim 1 wherein said luminescent material is calcium tungstate.

6. A screen as set forth in claim 1 wherein said polyesters are poly(methyl methacrylate) and poly(isobutyl methac-rylat'e), respectively.

References Cited in the file of this patent UNITED STATES PATENTS 2,310,740 Leavy Feb. 9, 1943 2,626,873 Allen Jan. 27, 1953 2,693,459 Fisk Nov. 2, 1954 2,694,153 Reuter Nov. 9, 1954 2,719,139 Wicklatz Sept. 27, 1955 2,728,703 Kierrnan et al. Dec. 27, 1955 2,740,050 Schultz Mar. 27, 1956 

1. A LUMINESCENT SCREEN COMPRISING A SUPPORT AND A LAYER OF LUMINESCENT MATERIAL BEARING A PROTECTIVE LAYER 